1. Field of the Invention
This invention relates to an improved process for the preparation of organoalkoxysilanes, which contain organic functional groups derived from organic acids or hydrogen cyanate. The process is based on the known reaction of alkali or alkaline earth metal salts or ammonium salts of organic acids or metal cyanates with haloalkylalkoxysilanes in the presence of phase transfer catalysts.
2. Description of Related Art
The use of guanidinium salt as a phase transfer catalyst for the preparation of organoalkoxysilanes from salts of organic acids and haloalkylalkoxysilanes is known. In particular, U.S. Pat. No. 5,950,150 to Simonian et al. discloses this phase transfer catalyst and its use in certain reactions related to producing isocyanurate and carboxylate silanes. U.S. Pat. No. 5,950,150 discloses methods to minimize the increase in temperature from the evolution of heat from a batch reaction to produce organoalkoxysilanes from an alkali metal salt or ammonium salt of an organic acid or a metal cyanate, but does not disclose methods for practicing a continuous or batch reaction or methods to produce the organoalkoxysilane product from the base organic acids or hydrogen cyanate.
Furthermore, U.S. Pat. No. 5,950,150 does not disclose that the preferred embodiment of the phase transfer catalyst, hexaethylguanidinium chloride, is commercially available only as an aqueous solution or dissolved in chlorobenzene or dichlorobenzene solvent. Either the aqueous or solvent form of the catalyst present substantial hindrances to the successful practicing of the process. Water reacts with the haloalkylalkoxysilane reactant and/or the organoalkoxysilane product to form by-product siloxanes, which decrease product yield. Additionally, the alcohol formed by the water hydrolysis reaction deactivates the guanidinium catalyst, which stops the reaction to form the organoalkoxysilane product. Chlorobenzene and dichlorobenzene are considered hazardous to humans since they are suspected carcinogens, moderately toxic, strong central nervous system depressants, and experimental teratogens. If introduced into a continuous or batch process with the phase transfer catalyst, chlorobenzene or dichlorobenzene would either enter into a distillation light ends recycle loop or become a waste stream. If the chlorobenzene or dichlorobenzene enters a continuous lights ends recycle loop, it will build to a high concentration and would have to be diverted to a waste stream. Either way, chlorobenzene or dichlorobenzene would become an undesired hazardous waste from the process.
The use of tetraalkylammonium and hexaalkylguanidinium salts as phase transfer catalysts in the preparation of various polymers is known. In particular, U.S. Pat. No. 5,132,423 discloses the reaction of bisphenol salts with halo- or nitro-substituted phthalimides in an organic medium to produce bisimides which, upon conversion to dianhydrides and reaction with diamines, form polyetherimides. U.S. Pat. No. 5,229,482 discloses a similar phase transfer catalyst reaction of bisphenol salts with halo- or nitro-substituted bis(phthalimido) derivatives of aromatic diamines or with similar compounds, resulting in the direct formation of polyetherimides and other polyether polymers. The phase transfer catalysts employed according to U.S. Pat. Nos. 5,132,423 and 5,229,482 are guanidinium salts and especially hexaalkylguanidinium salts.
There are four general methods for the synthesis of an organoalkoxysilane compound:
1. Hydrosilylation of allyl- or vinyl-functional compound with trialkoxysilane.
2. Hydrosilylation of allyl- or vinyl-functional compound with trichlorosilane and subsequent alcoholysis.
3. Reaction of chloropropyltrimethoxysilane with a sodium or potassium salt of organic acid.
4. Reaction of an aminoalkysilane with a carbonate to form the product via the carbamate.
U.S. Pat. No. 3,517,001 discloses the first method and cites that isocyanurate-organosilanes have been prepared in the past by adding hydrosilanes to unsaturated isocyanates and more specifically allyl isocyanate in the presence of metal catalysts. This process is limited on a large scale because the hydrosilanes are expensive and the unsaturated isocyanates are typically highly toxic.
The first method suffers from many practical problems such as a slow hydrosilylation process, formation of by-products containing internal olefins, use of toxic and low flash point reagents such as trimethoxy- or triethoxysilane.
The second method is described in U.S. Pat. No. 4,281,145. It teaches that bis(3-trimethylsilylpropyl) fumarate can be made by the hydrosilylation of diallyl maleate with trichlorosilane and subsequent methoxylation of the trichlorosilyl compound to the desired product. Unfortunately, the handling of trichlorosilane is very dangerous due to the low boiling point, very high reactivity and toxicity of this material. Also the methanolysis process is difficult to control and produces large amounts of waste.
The third method is described in U.S. Pat. Nos. 3,607,901; 3,821,218, and 3,598,852. These methods are for synthesizing 1,3,5-tris(trialkoxysilylpropyl) isocyanurates. This process involves the reaction of potassium cyanate with chloropropylmethoxysilane in a polar aprotic solvent such as N,N dimethylformamide (DMF) which is toxic and difficult to remove. The reaction time is about 3 to 8 hours. The resulting material has purity about 70% and is highly colored.
Patents such as U.S. Pat. No. 5,218,133 and U.S. Pat. No. 4,880,927 disclose the fourth method and disclose that aminoalkylsilanes can be reacted with carbonates such as dimethyl carbonate in basic conditions which will form the carbamate. The carbamate is then neutralized and converted to the isocyanurate by a lengthy, high temperature, subatmospheric pressure cracking reaction which necessitates the use of a cracking catalyst such as aluminum triethoxide and a base catalyst such as sodium acetate.
U.S. Pat. No. 4,946,977 discloses the preparation of methacryloxypropyltrimethoxysilane by contacting potassium methacrylate with chloropropyltrimethoxysilane in the presence of tetraalkylammonium halides as phase transfer catalyst. The yield of the reaction is below 90% and the resulting product usually has a dark color due the thermal decomposition of the catalyst.
European patent application 483,480 describes the preparation of methacryloxypropyltrimethoxysilane with high yield by the contacting of potassium methacrylate with chloropropyltrimethoxysilane in the presence of 4-N,N-dialkylaminopyridine as a catalyst. 4-Dialkylaminopyridine is an effective catalyst in these processes but the use of dialkylaminopyridine is limited due to very high toxicity of these compounds.
It is therefore an object of the present invention to devise a continuous or batch process for performing the entire process for producing an organoalkoxysilane from an organic acid or a hydrogen cyanate and a haloalkylalkoxysilane. It is an object of this invention to provide a method for introducing commercial guanidinium salt as a phase transfer catalyst into the process without the generation of hazardous by-products. Furthermore, it is an object of this invention to provide a method for producing sulfur and phosphorus containing silanes and amide silanes in addition to the isocyanurate and carboxylate silanes of the prior art.